Exposure control method for digital camera

ABSTRACT

An apparatus and method for controlling exposure of a digital camera with high precision by setting a proper exposure amount by emitting auxiliary light during auto-focusing, detecting the luminance level of an object, and carrying out an exposure controlling operation. The apparatus includes means for detecting the illuminance of outside light; means for determining whether or not auxiliary light is required based on the detected illuminance of the outside light; means for detecting the luminance level of an object in an AE frame under the outside light prior to emitting the auxiliary light, and for detecting again the luminance level of the object in an AF frame under the auxiliary light after emitting the auxiliary light; means for determining whether or not the exposure amount of the AF frame has increased markedly than the exposure amount of the AE frame; and means for performing an exposure controlling operation based on the luminance level in the AE frame when it has been determined that the exposure amount of the AF frame has increased markedly in the determining step, or based on the luminance level in the AF frame when it has been determined that the exposure amount of the AF frame has not increased markedly in the determining step.

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and method forcontrolling exposure of a digital camera comprising an auxiliary lightemitter for temporarily illuminating an object with auto-focus auxiliarylight only during auto-focusing in order to provide light when theluminance level is not sufficient.

BACKGROUND ART

[0002] In general, in a digital camera, exposure of an object iscontrolled by setting the opening ratio of an aperture that is providedat a lens, the electronic shuttering amount of a CCD, which is animaging device, and the gain of an imaging signal output from the CCD.Many digital cameras use a hill-climbing auto-focusing device as anauto-focusing (hereinafter abbreviated as “AF”) device.

[0003] Here, auto-focusing means to automatically bring a taking lensinto focus with an object. In auto-focusing, the distance to the objectis measured to obtain focusing. Hill-climbing auto-focusing provides theprinciple of measuring distance in auto-focusing in which the locationof a lens where a high-frequency component having a luminance leveldetected by the CCD becomes a maximum is the location of light focus.

[0004] Therefore, when an object in a dark background does not havesufficient contrast, the position of the object can no longer bedetermined, so that the location of light focus cannot be adjusted. Toovercome this problem, when there is not sufficient contrast because theluminance level of the object in a dark background is low, auto-focusingis carried out. During the auto-focusing, an exposure controllingoperation which increases the gain of an imaging signal is carried out.However, when the luminance level is very low because the object is at aplace where there is very little light around it, such as in the dark,the luminance level remains insufficient even if the exposurecontrolling operation which increases the gain of an imaging signal iscarried out, so that it may not be possible to increase the precision ofthe auto-focusing. To overcome this problem, in recent digital cameras,an object is illuminated with AF auxiliary light in order to providelight when the luminance level is insufficient. In such digital cameras,an auxiliary light emitter for emitting AF auxiliary light is built inthe camera body, or is built in a flash unit for illuminating an objectto properly expose it when it is photographed in a dark environment oragainst the sun and is mounted to the camera body when necessary.

[0005] Here, the amount of exposure control (opening ratio of anaperture, electronic shuttering amount of a CCD, gain of an imagingsignal) is determined by comparing the luminance level of the objectwith a target value that is stored in RAM, which is a memory that needsto be sequentially subjected to a writing operation, and feeding backthe result of the comparison. Therefore, when the luminance level of theobject is not a proper level, the amount of exposure control alsobecomes an improper amount.

[0006] In related digital cameras, although the amount of exposurecontrol is updated based on the luminance level while the object isbeing monitored before photographing it, the amount of exposure controlis not updated during auto-focusing. In the case where auxiliary lightis emitted, that is, the background is bright, when the luminance levelof the object at the time of monitoring and that at the time ofauto-focusing are the same, no problems arise even if the amount ofexposure control is not updated. However, in the case where theluminance level of the object at the time of monitoring and that at thetime of auto-focusing are not the same, when the luminance level of theobject at the time of monitoring is used, the exposure is no longercontrolled by a proper value. For this reason, in related digitalcameras using AF auxiliary light, the precision of auto-focusing cannotbe increased. In order to mitigate this problem and perform a properexposure controlling operation even after emission of auxiliary light,it is necessary to compute the amount of exposure control again bydetecting again the luminance level of the object after the emission ofauxiliary light.

[0007] As mentioned above, when photographing an object with an AFcamera using a hill-climbing method, in order to focus an image when itis being photographed, the object needs to have sufficient contrast.There are many situations where the object does not have sufficientcontrast, such as when the outline of the object is not clear. When theobject cannot have sufficient contrast because its luminance level islow, the image can be focused by increasing the luminance level. Inother words, during auto-focusing, an exposure controlling operationwhich increases the gain of an imaging signal compared to that duringmonitoring is carried out. When sufficient contrast still cannot beprovided due to insufficient luminance level even after this exposurecontrolling operation has been carried out, the object is illuminatedwith AF auxiliary light of, for example, an LED, to increase theluminance level of the object for achieving focus. In a camera includingan auxiliary light emitter for emitting AF auxiliary light, when theamount of light of the place where the object exists is determined asbeing low during focusing, a controlling operation is carried out sothat auxiliary light is emitted.

[0008] In general, the operation modes to the time of recording an imagein a digital camera changes from a monitoring mode to a scan AF mode toa photographic mode in that order. Here, the monitoring mode is a modein which data of an image is displayed on a display section withoutstoring it on a storage medium (in DRAM) to monitor an object.

[0009] The scan AF mode is a mode for achieving focus before recordingan image. For example, in the scan AF mode, a shutter button is pressedhalfway. Here, in order to make it easier to perform AF, the aperture isbrought close to an open state and the focal depth is made small. Whenthe luminance level of an object is low, an exposure controllingoperation which increases the gain of an imaging signal is carried outto obtain sufficient contrast. The photographic mode is a mode forrecording an image.

[0010] Conventionally, during monitoring mode, an exposure controllingoperation (opening ratio of an aperture, electronic shuttering amount ofa CCD, and gain of an imaging signal) is carried out for each framerate. However, when the operation mode of the camera changes from themonitoring mode to the scan AF mode, an exposure controlling operationis not carried out again in the scan AF mode. Therefore, when auxiliarylight is emitted, the amount of exposure control does not change eventhough the luminance level changes, so that the exposure when auxiliarylight is emitted is no longer carried out by a proper amount. Thisresults in a problem in that the precision of the auto-focusing isreduced.

[0011] The present invention has been achieved to overcome problems suchas those mentioned above, and has as its object the provision of amethod for controlling exposure of a digital camera which, bycontrolling an exposure operation again even after emission of auxiliarylight for changing luminance, makes it possible to perform properexposure even when auxiliary light is emitted, so that the precision ofauto-focusing is increased.

DISCLOSURE OF INVENTION

[0012] To this end, the present invention provides a method forcontrolling exposure of a digital camera comprising means for emittingauxiliary light. The method comprises the steps of detecting theilluminance of outside light; determining whether or not the auxiliarylight is required based on the detected illuminance of the outsidelight; prior to emitting the auxiliary light, detecting the luminancelevel of an object in an AE frame in a monitoring mode under the outsidelight; after emitting the auxiliary light, detecting again the luminancelevel of the object in an AF frame in a scan AF mode under the auxiliarylight; determining whether or not the exposure amount of the AF framehas increased markedly than the exposure amount of the AE frame; andwhen it has been determined that the exposure amount of the AF frame hasincreased markedly in the determining step, performing an exposurecontrolling operation based on the luminance level in the AE frame, or,when it has been determined that the exposure amount of the AF frame hasnot increased markedly in the determining step, performing an exposurecontrolling operation based on the luminance level in the AF frame.

[0013] In the present invention, the exposure controlling step comprisescontrolling the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.

[0014] In the method for controlling exposure of a digital camera of thepresent invention, when the ratio between the exposure amount after theemission of the auxiliary light and the exposure amount prior to theemission of the auxiliary light exceeds a predetermined threshold value,the exposure amount in the AF frame is limited.

[0015] According to the present invention, there is provided an exposurecontroller of a digital camera comprising means for emitting auxiliarylight, the exposure controller comprising means for detecting theilluminance of outside light; means for determining whether or not theauxiliary light is required based on the detected illuminance of theoutside light; means for detecting the luminance level of an object inan AE frame in a monitoring mode under the outside light prior toemitting the auxiliary light, means for detecting again the luminancelevel of the object in an AF frame in a scan AF mode under the auxiliarylight after emitting the auxiliary light; means for determining whetheror not the exposure amount of the AF frame has increased markedly thanthe exposure amount of the AE frame; and means for performing anexposure controlling operation based on the luminance level in the AEframe when it has been determined that the exposure amount of the AFframe has increased markedly in the determining step, or based on theluminance level in the AF frame when it has been determined that theexposure amount of the AF frame has not increased markedly in thedetermining step.

[0016] In the exposure controller of a digital camera of the presentinvention, the exposure controlling means controls the opening ratio ofan aperture, the electronic shuttering amount of an imaging device, andthe gain of an imaging signal output from the imaging device.

[0017] In the exposure controller of a digital camera, when the ratiobetween the exposure amount after the emission of the auxiliary lightand the exposure amount prior to the emission of the auxiliary lightexceeds a predetermined threshold value, the exposure amount in the AFframe is limited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram showing the structure of a circuit of adigital camera control section used in an embodiment of the presentinvention.

[0019]FIG. 2 illustrates a timing of setting the exposure amount whenauxiliary light is emitted and not emitted.

[0020]FIG. 3 is a flowchart showing the flow of an exposure controloperation in a digital camera.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] Hereunder, a detailed description of one embodiment of a digitalcamera of the present invention will be given with reference to thedrawings. FIG. 1 is a block diagram showing the structure of a circuitof a digital camera control section used in an embodiment of the presentinvention.

[0022] As shown in FIG. 1, the digital camera of the embodimentcomprises a lens 1 for receiving light from an object, an aperture 2 foradjusting exposure, and a CCD 3, which is an imaging device, where alight image of the object from the lens 1 is focused. A holding/gaincontrol circuit 4 which sample holds an imaging signal output from theCCD 3 and which controls the gain of the imaging signal is connected toan output terminal of the CCD 3. An A/D converter 5 for subjecting theimaging signal whose gain has been controlled to analog-to-digitalconversion is connected to this holding/gain control circuit 4.

[0023] An image signal processing circuit 6 for variously processing theimaging signal is connected to an output terminal of the AD converter 5.A luminance level detector 7 for detecting as a luminance level anintegral value of a luminance signal from a predetermined image area andextracted from an image signal is connected to the image signalprocessing circuit 6.

[0024] A microcomputer 8 for controlling the entire operation of thedigital camera comprises an exposure control circuit 9, a device controlcircuit 10, and an operational circuit 14. The exposure control circuit9 is connected to an output terminal of the luminance level detector 7.Based on the luminance level detected by the luminance level detector 7,the exposure control circuit 9 determines whether or not proper exposureis being carried out. Based on the obtained data on the result of thedetermination, the exposure control circuit 9 computes control dataregarding each device, that is, data on the opening ratio of theaperture 2, data on the electronic shuttering amount of the CCD 3, andgain data from the holding/gain control circuit 4. The exposure controlcircuit 9 has RAM (not shown). Based on the luminance level detected bythe luminance level detector 7, various pieces of data that are requiredwhen an exposure controlling operation is carried out are written toRAM. In addition, the various pieces of data are read out from RAM.

[0025] The device control circuit 10 for outputting a drive signal ofeach device is connected to an output terminal 9 a of the exposurecontrol circuit 9. An aperture driver 11 is connected to a first outputterminal 10 a of the device control circuit 10. The aperture 2 is suchas to be driven by an output signal from the aperture driver 11. Atiming generation circuit 12 for outputting a timing signal thatcontrols the electronic shutter of the CCD 3 is connected to a secondoutput terminal 10 b of the device control circuit 10. The holding/gaincontrol circuit 4 is connected to a third output terminal 10 c of thedevice control circuit 10.

[0026] The operational circuit 14 is connected to an output terminal 9 bof the exposure control circuit 9. From the luminance level, that is,the integral value of a luminance signal from a predetermined image areaand extracted from an image signal, computed by the luminance leveldetector 7, and the control data of each device that is computed basedon the luminance level by the exposure control circuit 9, theoperational circuit 14 makes a computation to determine whether or notemission of AF auxiliary light is required. An LED 15 for emitting AFauxiliary light is connected to an output terminal of the operationalcircuit 14, and emits light when necessary.

[0027] Next, a description of the exposure controlling operation of thedigital camera of the present invention will be given based on theabove-described structure. In the embodiment, the exposure controllingoperation is considered as being carried out when the luminance level ofan object is changed by emitting auxiliary light in the scan AF mode.More specifically, the luminance level changes from monitoring toauto-focusing after emission of auxiliary light. Even at the time of theauto-focusing, the luminance level of the object is detected again (theCCD takes in light) for controlling exposure, thereby increasing theprecision of the auto-focusing. Auxiliary light is only emitted when theamount of light of the surrounding is small the luminance level is low,so that it is not emitted when the amount of light of the surrounding islarge.

[0028]FIG. 2 shows a sequence of the temporal flow of the exposurecontrolling operation when auxiliary light is emitted.

[0029]FIG. 2 illustrates from the top (i) sequence of change in theoperation mode of the camera, (ii) on and off states of the LED 15 foremitting auxiliary light, (iii) the CCD 3 takes in (detects) light, (iv)computation at the image signal processing circuit 6, and (v)computation of the exposure amount. Here, each number along thehorizontal axis represents frame rate numbers (hereinafter referred toas the “frame numbers”). As shown in FIG. 2, the frame number where thelast computation of the exposure amount is carried out in a monitoringmode a1 is 0.

[0030] First, detection frames of a liquid crystal display will bedescribed. Since up to frame number 0 the computation of the exposureamount is carried out in the monitoring mode a1, the luminance level isdetected at a luminance level detection frame (hereinafter referred toas the “AE frame”) that is such as to allow proper exposure during themonitoring mode in order to compute the exposure amount. Here, the AEframe refers to the area of the entire liquid crystal display. Since thecomputation of the exposure amount from frame number 1 onwards iscarried out in a scan AF mode a2, ordinarily, the luminance level isdetected at a luminance level detection frame (hereinafter referred toas the “AF frame”) that is such as to allow proper exposure during thescan AF mode in order to compute the exposure amount. Here, the AF framerefers to an area of a portion of the screen central portion of theentire area of the liquid crystal display.

[0031] When the camera operation mode changes from the monitoring modea1 to the scan AF mode a2, the detection frame changes from the AE frameto the AF frame as mentioned above. This is because, when auto-focusingis to be carried out, it is necessary to increase the precision of theauto-focusing by previously providing a detection frame which is thoughtto have a high possibility of having an object in an angle of view andby detecting the luminance level inside the detection frame. Inaddition, when zooming is carried out, also considering, for example,the range of illumination of the auxiliary light, the AF frame changesto an optimal detection frame for carrying out auto-focusing. However,in the case where the exposure controlling operation is carried out by asuitable exposure amount of the AF frame, when the luminance level of animage increases markedly compared to the luminance level in themonitoring mode, the AE frame is used as the detection frame as in themonitoring mode.

[0032] To frame number 1 where the exposure amount is computedimmediately after the operation mode has changed to the scan AF mode,operation flows as indicated by arrow A shown in FIG. 2, that is, (iii)the CCD 3 takes in light, (iv) computation is carried out by the imagesignal processing circuit 6, and (v) by the operation flow indicated byarrow A, the exposure amount is computed based on the luminance level atframe number 1 where the CCD has taken in light {(1) the exposure amountis determined}. For this reason, the exposure amount is computed by theluminance level before auxiliary light is emitted (when auxiliary lightis not emitted). In other words, since, at frame number 1 where theexposure amount is computed, the operation mode has already changed tothe AF mode, by selecting the AF frame, the luminance level of theobject is detected to compute the exposure amount.

[0033] The exposure amount computed at frame number 1 is reflected whenthe CCD has taken in light at frame number 4 (which is shaded) by anoperation flow indicated by arrow B shown in FIG. 2 {(2) the exposureamount is reflected}. When the CCD takes in light at frame number 4,auxiliary light has already been emitted, so that, by an operation flowindicated by arrow C, the exposure amount is computed (at frame number4) {(3) the exposure amount after the emission of auxiliary light isdetermined}. At this time, since auxiliary light is emitted, theluminance level of an image is increased. At this time, by selecting theAF frame and making a detection, the luminance level is detected tocompute the exposure amount.

[0034] When the amount of change in the exposure due to the emission ofauxiliary light is greater than a predetermined threshold value, theexposure amount is limited so that a sudden change in the luminancelevel of the image is restricted. This limiting operation is carried outwhen the amount of change in the exposure exceeds a predeterminedthreshold value. Here, the predetermined threshold value is previouslyset in RAM (not shown) in the exposure control circuit 9. By anoperation flow indicated by arrow D, the exposure amount that has beencomputed in this way is reflected at frame number 7 where the CCD takesin light {(4) the exposure amount is reflected}. At frame number 7 wherethe CCD takes in light, by setting the exposure amount after theemission of auxiliary light, the object has sufficient contrast, so thatthe precision of the auto-focusing can be increased.

[0035] Even if the exposure amount after the emission of auxiliary lightis still not the proper value, by repeating these steps, the precisionof the exposure is increased, so that the precision of the auto-focusingcan be increased even more.

[0036]FIG. 3 is a flowchart showing the flow of the steps of theexposure controlling operation of the digital camera.

[0037] In FIG. 3, first, the illuminance of outside light is detected bythe digital camera (Step S1), and a determination is made as to whetheror not auxiliary light is required (Step S2). When it is determined thatauxiliary light is required (when the answer is “yes” in Step S2), theCCD takes in a detected value based on the exposure amount that has beencomputed at a previous frame (Step S3), and the image signal processingcircuit 6 makes a computation (Step S4). Thereafter, auxiliary light isemitted (Step S5), and the exposure amounts at the AE frame and the AFframe are computed (Step S6).

[0038] Next, in Step S7, a determination is made as to whether or notthe exposure amount of the AF frame has markedly increased compared tothe exposure amount of the AE frame. When it has been determined thatthe exposure amount of the AF frame has not markedly increased, “AFframe exposure” is selected (Step S8). When it has been determined thatthe exposure amount of the AF frame has markedly increased, “AE frameexposure” is selected (Step S9), and the operation proceeds to Step S10.In Step S10, a determination is made as to whether or not the amount ofchange in the exposure has exceeded a threshold value. When it has beendetermined that the amount of change in the exposure has exceeded thethreshold value, the exposure amount is limited (Step S11). Theoperation proceeds to Step S12. On the other hand, when it has beendetermined that the amount of change in the exposure has not exceededthe threshold value in Step S10, the operation proceeds to Step 12without limiting the exposure amount.

[0039] Next, in Step S12, the CCD 3 takes in the detected value of theAF frame, and the image signal processing circuit 6 makes a computation(Step S13). Thereafter, the exposure amounts of the AE frame and the AFframe are computed (Step S14).

[0040] Next, in Step S15, a determination is made as to whether or notthe exposure amount of the AF frame has markedly increased compared tothe exposure amount of the AE frame. When it has been determined thatthe exposure amount of the AF frame has not markedly increased, “AFframe exposure” is selected (Step S16). When it has been determined thatthe exposure amount of the AF frame has not markedly increased, “AEframe exposure” is selected (Step S17), and the operation proceeds toStep S18. In Step S18, a determination is made as to whether or not theamount of change in the exposure has exceeded the threshold value. Whenit has been determined that the amount of change in the exposure hasexceeded the threshold value, the exposure amount is limited (Step S19).The operation proceeds to Step S20 to determine whether or not theamount of change in the exposure is a proper amount. On the other hand,when a determination is made that the amount of change in the exposurehas not exceeded the threshold value in Step S18, the operation proceedsto Step 20 without limiting the exposure amount.

[0041] In Step S20, when the amount of change in the exposure is not theproper amount, the operation returns to Step S12, and the Steps S12 toS20 are repeated. In Step S20, when the amount of change in the exposureis the proper amount, a proper exposure controlling operation is carriedout in Step S21 based on the exposure amount obtained in Step S14.

[0042] On the other hand, when it is determined that auxiliary light isnot required (when the answer is “no” in Step S2), the CCD 3 takes in adetected value (Step S22), and the image signal processing circuit 6makes a computation (Step S23). Thereafter, the exposure amount of theAE frame and the exposure amount of the AF frame are computed (StepS24), and, based on the exposure amounts obtained in Step S24, theexposure controlling operation is carried out (Step S25).

[0043] As described in detail above, according to the present invention,auxiliary light is emitted during auto-focusing to detect the luminancelevel of an object. Based on the luminance level, the exposure amount isdetermined. Based on the exposure amount, the exposure controllingoperation is carried out. Therefore, the exposure controlling operationis properly carried out, so that the precision of the auto-focusing canbe increased.

[0044] In addition, according to the present invention, auxiliary lightis emitted during auto-focusing to detect the luminance level of anobject. Based on the luminance level, the exposure amount is determined.Based on the exposure amount, the exposure controlling operation iscarried out. Therefore, pictures can be taken in dark places.

[0045] Further, according to the present invention, a determination ismade as to whether or not the exposure amount of the AF frame hasmarkedly increased than the exposure amount of the AE frame. When it isdetermined that it has markedly increased in the determining step, theexposure controlling operation is carried out based on the luminancelevel of the AE frame. Therefore, it is possible to carry out theexposure controlling operation without putting the camera in the scan AFmode, so that the exposure controlling operation can be quickly carriedout.

[0046] When auto-focusing is carried out, auxiliary light is emitted todetect the luminance level of an object. Therefore, even if a userintentionally sets the exposure amount outside the range of properexposure amounts in, for example, manual mode, the user can effectivelycarry out auto-focusing by detecting the luminance level of the objectagain.

1. A method for controlling exposure of a digital camera comprisingmeans for emitting auxiliary light, the method comprising the steps of:detecting the illuminance of outside light; determining whether or notthe auxiliary light is required based on the detected illuminance of theoutside light; prior to emitting the auxiliary light, detecting theluminance level of an object in an AE frame in a monitoring mode underthe outside light; after emitting the auxiliary light, detecting againthe luminance level of the object in an AF frame in a scan AF mode underthe auxiliary light; determining whether or not the exposure amount ofthe AF frame has increased markedly than the exposure amount of the AEframe; and when it has been determined that the exposure amount of theAF frame has increased markedly in the determining step, performing anexposure controlling operation based on the luminance level in the AEframe, or, when it has been determined that the exposure amount of theAF frame has not increased markedly in the determining step, performingan exposure controlling operation based on the luminance level in the AFframe.
 2. A method for controlling exposure of a digital cameraaccording to claim 1, wherein the exposure controlling step comprisescontrolling the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.
 3. A method for controlling exposure of adigital camera according to claim 1, wherein, when the ratio between theexposure amount after the emission of the auxiliary light and theexposure amount prior to the emission of the auxiliary light exceeds apredetermined threshold value, the exposure amount in the AF frame islimited.
 4. An exposure controller of a digital camera comprising meansfor emitting auxiliary light, the exposure controller comprising: meansfor detecting the illuminance of outside light; means for determiningwhether or not the auxiliary light is required based on the detectedilluminance of the outside light; means for detecting the luminancelevel of an object in an AE frame in a monitoring mode under the outsidelight prior to emitting the auxiliary light, and for detecting again theluminance level of the object in an AF frame in a scan AF mode under theauxiliary light after emitting the auxiliary light; means fordetermining whether or not the exposure amount of the AF frame hasincreased markedly than the exposure amount of the AE frame; and meansfor performing an exposure controlling operation based on the luminancelevel in the AE frame when it has been determined that the exposureamount of the AF frame has increased markedly in the determining step,or based on the luminance level in the AF frame when it has beendetermined that the exposure amount of the AF frame has not increasedmarkedly in the determining step.
 5. An exposure controller of a digitalcamera according to claim 4, wherein the exposure controlling meanscontrols the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.
 6. An exposure controller of a digital cameraaccording to claim 4, wherein, when the ratio between the exposureamount after the emission of the auxiliary light and the exposure amountprior to the emission of the auxiliary light exceeds a predeterminedthreshold value, the exposure amount in the AF frame is limited.